Detectors are known using a semiconductor crystal, for example of cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe), within which an electric field is applied by an anode and a cathode disposed on opposite sides of the crystal.
The ionizing radiation interacting with the crystal gives rise to charge separations, that is to say the formation of electron-hole pairs, the electron and the hole, initially formed at the same location, rapidly migrating under the effect of the electric field in opposite directions. It is then possible to collect the electrons and/or the holes, that is to say to form an electrical signal from the charges flowing towards the anode and/or from the charges flowing towards the cathode, the electrical signal thus measured reflecting the interaction of the ionizing radiation in the semiconductor.
It should be specified that in addition to gamma radiation, alpha radiation, beta radiation, X-rays and neutrons may be detected using such semiconductor detectors.
The known semiconductor detectors with two-dimensional spatial resolution give better perspectives with regard to spatial resolution than the more widespread detectors based on the scintillation of a crystal or of a plastic in reaction to photons, X- or gamma rays.
Semiconductor detectors with spatial resolution are in particular capable of enabling medical imaging to be performed, based on the use of a gamma radiation emitting radioactive product injected into the patient or into an animal and selectively fixing in its body.
Thus semiconductor detectors are known that use a cathode on one face of the semiconductor crystal, and a rectangular array of pixels forming anodes on the other face, each pixel defining both an X-coordinate and a Y-coordinate.
Such a structure is referred to as an “n2” structure, n designating the number of anode pixels aligned with one of the sides of the semiconductor, which is considered to be square. It is complex to implement when the value of n increases, since it is necessary to put in place contacts and electrical conductors to extract the signal from each pixel, and to install a high number of channels for processing the signals.
Thus, this solution becomes of low practicability when the needs for spatial resolution and for detection area increase. However, in medical matters, the spatial resolution desired is less than a millimeter, and it is necessary to have available a field of view of several square centimeters (field 5×5 cm2 for applications on a small animal). Furthermore, it is desirable for the detectors and the electronics they carry to occupy a compact volume, so as to make them easier to manipulate in a restricted space, such as in the field of medical imaging, in which it is necessary to place the detector as close as possible to the organ under study, whereas the access to that organ may be awkward.
Structures enabling the installation of a fewer number of electronic channels while maintaining good spatial resolution are thus sought.
Structures designated under the generic term of “2n” structures are known in particular.
In the case of the semiconductors for which the electrons and the holes do not migrate at the same speed, which is the case for CdZnTe or for CdTe, these structures generally comprise a cathode on one face of the semiconductor crystal, and use two series of electrodes on the opposite face, the electrodes extending along one or other of the dimensions of the surface of the crystal. The designation “2n” thus means that although it is desired to produce an imager with n2 pixels, it is not a matter of using n2 pixelized anodes on the surface of the semiconductor material, but n elongate electrodes forming a first series of electrodes and n other electrodes, also elongate, forming a second series of electrodes.
The electrodes of the first series define the X coordinate and are deposited directly on the semiconductor material. These are collecting electrodes, since they collect charge carriers migrating in the detector. When those carriers are electrons, the collecting electrodes are anodes.
The electrodes of the second series are disposed transversely to the collecting electrodes, and define the Y-coordinate. These electrodes are not in electrical contact but are capacitively coupled with the semiconductor crystal. They do not collect charge carriers, but produce a signal induced by the movement of charge carriers in the detector material, including a maximum and a minimum potential. They are thus referred to as non-collecting electrodes.
In U.S. Pat. No. 6,037,595, such a “2n” system, called “cross-strip detector” is disclosed.
The electrodes of a first series of electrodes are constructed by the use of localized anodes, deposited on the semiconductor material and linked together in columns by conductive cables passing at a distance from the semiconductor material.
Electrodes originally put in place on the surface of the semiconductor to protect the localized anodes in relation to electrostatic effects induced remotely by the movement of the electrons and holes in the semiconductor volume are used to form the second series of electrodes. They are organized in the form of lines, and are also linked by conductive cables passing at a distance from the semiconductor material. As these electrodes do not collect electrons but only measure induced effects, they are referred to as “non-collecting”.
A variant described in this patent uses a layer of insulating material to separate the detector material and the non-collecting electrodes; this layer of insulating material is deposited on the face of the detector. The non-collecting electrodes are then either formed on the insulating layer, or are formed on a separate substrate, which is then mounted on the insulating layer.
PCT Pat. Pub. No. WO2008/054862 also describes a detector having a “2n” type structure and comprising a volume 2 of semiconductor material, a cathode 4 on one face of that volume, an anode structure 6 on the opposing face, the latter comprising a first series of conductor bands 8 (collecting anodes) on the face of the semiconductor material and a second series of conductor bands 12 (non-collecting anodes) separated from the collecting anodes by a layer of insulating material 26, which may be AlN, Al2O3 or Si3N4. The insulating material may be deposited by sputtering or evaporation. The sputtering may lead to a heterogeneous insulating layer being obtained, strewn with air bubbles, and of uneven thickness. The evaporation only takes place at high temperature and is thus not without risk for the semiconductor crystal whose properties are generally very temperature-sensitive. The manufacturing methods proposed are not satisfactory therefore.
Furthermore, to ensure that the electrons are actually collected by the localized anodes, a focusing grid kept at a potential intermediate between the cathode and anode potentials is sometimes used to direct the electrons. This focusing grid may also be referred to as a non-collecting electrode since it is separated from the semiconductor by an insulating layer and collects no electron.
From U.S. Pat. Pub. No. 2002/0036269 in particular, a coincidence detection device is known comprising at least two semiconductor detector crystals, each crystal bearing pixellated anodes on a first face and segmented cathodes on a second face, each pixellated anode being connected to an electronic channel for collecting signals representing the energy of the photons interacting in the crystal, each segmented cathode being connected to an electronic channel for collecting coincidence trigger signals. In an embodiment illustrated in FIGS. 3A and 3B of this published patent application, a non-collecting focusing grid 212 is formed between the pixellated anodes. An insulating layer 210 (of grid form) extends between the first face of the crystal and the focusing grid. As indicated [0068], the insulating material can be painted, sprayed, deposited, chemically passivated bonded, bonded or vaporized onto the face of the crystal. As explained earlier, these methods are not fully satisfactory.
In another embodiment illustrated in FIG. 7 of U.S. Pat. Pub. No. 2002/0036269, the detector includes as earlier a crystal and pixellated anodes. Furthermore, a printed circuit board bearing pixellated contacts (612) and a focusing grid (616) is juxtaposed against the detector: the contacts (612) are bonded to the pixellated anodes (606) of the detector using a conducting adhesive. The space between the contacts (612), the anodes (606) and the gird (616) is filled with an insulating polymer material (622) in the liquid state using a wick ([0098]). This insulating layer 622 is formed during or after the assembly of the detector 602 and the printed circuit board 610 ([0099]). The insulating material brought to a high temperature then enters into contact with the detector, which may damage the latter. Furthermore, the operation of injecting insulating material into the narrow space destined to receive it is awkward and difficult to implement industrially. Separating the detector and the printed circuit board with the aim of facilitating the distribution of the insulating material would lead to an insulating layer that is too thick being obtained.
In the technical article “Single-sided CZT Strip detectors” (J. Macri et al., IEEE, Vol 51, No. 5, October 2004), instead of using a focusing grid, the non-collecting electrodes are themselves brought to a potential intermediate between the cathode and anode potentials.
In both cases, the installation of the conductor cables, outside the plane of the localized anodes, considerably complicates the manufacture, in particular when the needs for spatial resolution or area increase. The use of different potentials for the two series of anodes or for the focusing grid also constitutes a difficulty, since surface currents appear when the surface state of the semiconductor or of the other materials used is not perfect.
Furthermore, a production method according to a variant of U.S. Pat. No. 6,037,595 requires the use of a dielectric material which can be deposited at a temperature capable of being withstood by the semiconductor material on which the deposition is made. Failing this, the semiconductor material is deteriorated. This temperature condition is particularly limiting where the semiconductor material comprises CdTe or CdZnTe, such materials degrading (loss of spectro properties, in particular) as soon as the temperature exceeds 80° C.
From European Pat. No. EP1739458 a detector is also known using two series of electrodes disposed in two separate planes parallel to the surface of the semiconductor, still on the same side thereof, the electrodes being in the form of strips and both series of electrodes being separated by an electrically insulating material. The thickness of the latter is adjusted to obtain a compromise between detection of the induced signal and insulation of the non-collecting electrodes.
During the manufacture of such a device, the deposition of an insulating material on the surface of the semiconductor is awkward, since at low temperature, adhesion is liable to be insufficient and the thickness poorly controlled, and at high temperature, the semiconductor is liable to be damaged.
Furthermore, it is naturally desirable to improve the ratio between the signal measured by the non-collecting electrodes and the noise to which the latter are subjected, in particular the noise appearing by capacitive effect, on account of the collection of charges by the collecting anodes situated nearby.